In a boiling water reactor, the fuel is in the form of fuel rods, each comprising a stack of pellets of a nuclear fuel arranged in a cladding tube. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a definite symmetrical pattern, i.e., 2, so-called lattice. Usually, the fuel rods are arranged in an orthogonal lattice, that is, each fuel rod is included in two rows of fuel rods arranged perpendicular to each other. By a fuel rod position is meant a position in the lattice. The fuel rods are retained at the top by an upper retaining member, for example a top tie plate, and at the bottom by a bottom tie plate. To keep the fuel rods spaced apart from each other and to prevent them from bending or vibrating when the reactor is in operation, a number of spacers are distributed along the fuel bundle in the longitudinal direction of the fuel assembly. A fuel rod comprises one or more fuel bundles, each extending along the main part of the length of the fuel assembly and being surrounded by a substantially square fuel channel.
The core is immersed into water which serves both as a coolant and as a neutron moderator. During operation, the water flows upwardly in the fuel assembly, whereby part of the water is transformed into steam. As a moderator, steam is inferior to water because of the lower density of steam. During operation, therefore, the moderation of the upper part of the feul assembly is not as good as that in its lower part. If the fuel is distributed uniformly along the longitudinal direction of the fuel assembly, a lower moderator/fuel ratio is obtained in the upper part compared with that in the lower part. A fuel assembly with a non-uniform moderator/fuel ratio has an inferior reactivity during operation, which contributes to an inferior fuel economy compared with a fuel assembly which has a uniform moderator/fuel ratio. To obtain a more uniform moderator/fuel ratio when the reactor is hot, the amount of fuel should thus be smaller and the lattice space, that is, the free space between the fuel rods, should be larger in the upper part of the fuel assembly than in its lower part. For a certain fuel design, a more uniform moderator/fuel ratio may be achieved, for example, by selection of the diameter of the fuel rods, the distance between the fuel rods, and the number of fuel rods.
One way of increasing the moderator/fuel ratio in the upper part of the fuel assembly is to replace some of the fuel rods in the lattice with part-length fuel rods. Part-length fuel rods extend from the bottom tie plate towards the top tie plate but terminate somewhat below the top tie plate in contrast to top full-length fuel rods which extend from the bottom tie plate to the top tie plate. U.S. Pat. No. 5,229,068 discloses a fuel assembly in which the majority of the fuel rods are full-length rods and a minor number of the fuel rods are part-length rods. All the fuel rods in the fuel assembly, that is, both the full-length and the part-length rods, are arranged in an orthogonal lattice in which each rod is included in two rows of fuel rods perpendicular to each other. The distance between two adjacent fuel rods is the same irrespective of whether the fuel rod is a full-length rod or a part-length rod.
One disadvantage of having part-length fuel rods in certain fuel rod positions is that the lattice in the upper part of the fuel assembly becomes uneven with large openings. Because of the low flow resistance of the openings, part of the water which would otherwise have flown between the fuel rods will instead flow in these openings. This results/sin inferior cooling of certain fuel rods and in increased risk of dryout and possible fuel damage.
Another disadvantage is that the total length of all the fuel rods is reduced compared with a fuel assembly with full-length fuel rods in all fuel rod positions. This results in an increase of the linear load (power/unit of length) of the fuel rods, which in turn increases the temperature of the fuel and also increases the risk of PCI damage (PCI=Pellet-Clad Interaction) to the fuel while at the sane time increasing the emission of fission gases.
An additional disadvantage are the peaks in the neutron flux which arise at the top of the part-length fuel rods. The neutron flux peaks arise because the top part of the part-length fuel rods includes parts which neither contain any uranium nor any water. These parts are made up of the fission gas space and the top plug. In these uranium-free and moderator-free parts, no neutron absorption takes place, thus providing a surplus of neutrons which are instead absorbed in the uppermost fuel pellets in the part-length fuel rod and hence cause a power peak.